DESCRIPTION: (Adapted from Investigators' Abstract) On April 2-3, 1990 an International Workshop on Chromosome 21 was held in Bethesda, Maryland, with the support of the Human Genome Center. One of the actions of this workshop was to establish a joint effort to screen the St. Louis YAC library constructed in the laboratory of Dr. Maynard Olson for YACs specific to Chromosome 21. Since this library has been transferred to the laboratory of Dr. David Patterson at the Eleanor Roosevelt Institute in Denver, Dr. Patterson agreed to carry out this screening if appropriate support could be obtained. Any researcher working on human chromosome 21 may participate in the Joint Effort. Screening will be done by PCR based technology. The requesting investigators will be responsible for providing primers to the screening laboratory. One specific focus of the initial screening effort will be to isolate a set of reference YACs corresponding to the 23 ordered, well spaced reference probes designated by the International Workshop. Investigator requests will be handled in turn, with each investigator specifying one PCR primer set per turn. Investigators will be expected to make all YACs immediately available to any members of the Joint Effort. A Newsletter, FAX on the YACs, will be established to communicate with the Joint Effort. A Scientific Advisory Committee has been established to oversee the operation. It is anticipated that this will make screening of this unique YAC library available to all interested researchers fairly and efficiently.